Heat transfer fluids (also denoted as “thermal fluids”) are widely used for temperature control in manufacturing facilities. The ability of a heat transfer fluid to resist degradation at elevated temperatures is referenced through a performance property denoted as “thermal stability”. As a heat transfer fluid undergoes degradation, both volatile materials (which “boil” or evaporate from the fluid) and “heavy” materials (also denoted as “high boilers” or “residue”) are formed within the heat transfer fluid. Such heavy components elevate the heat transfer fluid's viscosity and thereby lead to an increase in film temperature in the portion of heat transfer fluid denoted as the “film” which exists at the interface of the heat transfer fluid and a high temperature (respective to the temperature of the fluid) surface; any such differential in film temperature above the established maximum operating temperature of the heat transfer fluid augments the rate of degradation in the heat transfer fluid as a whole insofar as the film portion intermixes with the remainder of the heat transfer fluid. Further, tars or polymers formed through the degradation of a heat transfer fluid tend to darken the fluid and ultimately deposit on surfaces in the system; these deposits are detrimental to system efficiency and potentially lead to system failure. Significantly, degraded heat transfer fluid must, therefore, be periodically replaced with fresh or recycled heat transfer fluid.
Commonly used components of thermal fluids include partially hydrogenated terphenyls (PHTs) found for example in Therminol® 66 marketed by Eastman, and terphenyls, the primary component in Therminol® 75 also marketed by Eastman. Therminol® 66 also contains partially hydrogenated quaterphenyls while Therminol® 75 contains the same components and isomer ratios but in a non-hydrogenated form.
It is known that thermal fluids made primarily with terphenyls exhibit higher thermal stability than thermal fluids made with PHTs. However, thermal fluids made only with mixed terphenyls, such as Therminol® 75, suffer from the fact that they have a freezing point which prevents them from being introduced into a system without significant heating so as to make them fluid and pumpable. On the other hand, while PHTs such as used in Therminol® 66 provide a thermal fluid which is liquid and therefore pumpable at room temperature, e.g., 72° F. (22.2° C.), it suffers from the disadvantage that it has reduced thermal stability resulting from the saturated rings created from hydrogenation.
Regardless of whether the thermal fluid is terphenyl based or PHT based, the fluids are expensive. Over time, the thermal fluids degrade due to various reasons such as high temperatures. Degradation leads to the formation of light ends and tars. In the case of PHT based fluids, ultimately, they degrade to the point where significant amounts revert back to terphenyls along with generating tars and polymers. It is the formation of these tars and polymers which cause the viscosity to increase.